The presence of a massive white cap on Uranus may appear to be disturbing, yet as planetary scientists are learning, this is the thing that a drawn-out summer looks like on the remote ice giant.
Ice giants Uranus and Neptune have water-rich interiors covered with hydrogen, helium, and methane, the last of which gives these external planets their particular cyan complexion. In contrast to Earth, where seasons last only a couple of months, Neptune and Uranus experience seasons that keep going for quite a long time, bringing about weird and intense atmospheric phenomena.
Recently, the Outer Planet Atmospheres Legacy (OPAL) program has released new images that highlight an evolving atmospheric event on both ice giants, namely an extended white cap over Uranus’ north pole and a new dark vortex on Neptune. The new data, captured during the autumn of 2018, are providing important new insights into the seasonal variations on both Neptune and Uranus.
The large white cap strewn over the north pole of Uranus is particularly dramatic. The likely cause of this feature has to do with the planet’s unique tilt, which causes sunlight to shine directly onto the north polar regions for an extended period of time during the summer. It’s currently mid-summer at Uranus’ north pole, resulting in the protracted white cap.
Amy Simon, a scientist at NASA’s Goddard Space Flight Center who leads the OPAL mission told Gizmodo, “The yearly observations are helping us to understand the frequency of storms, as well as their longevity. That’s important because these planets are quite far from the Sun, so this will help constrain how they are forming and more about the internal heat and structure of these planets. Most of the extrasolar planets that have been found are this size of a planet, though at all sorts of distances from their parent stars.”
Leigh Fletcher, an astronomer at the University of Leicester said, “The November 2018 image of Uranus occurs at a time 10 years after the equinox, when the northern hemisphere was just emerging into spring sunlight after spending decades in polar winter. Back in 2007, there didn’t appear to be anything like this polar cap over the springtime pole. But as time progressed, a reflective band—whitish against Uranus’ blue hues—began to appear encircling the north pole. And now, 10 years on, that band has turned into a thick polar cap of aerosols that are hiding the deeper polar region from view.”
“It’s a spectacular example of seasonal change on this ice giant, with the aerosol cap evolving as spring becomes summer. The exact causes of these aerosol changes, he said, remain a mystery, with possibilities including warming temperatures, unusual chemistry, some large-scale atmospheric circulation pattern, or a combination of all these.”
“Thankfully we’re not too far away from having an answer, as the James Webb Space Telescope will be able to diagnose the temperatures and chemistry responsible for these reflectivity changes that Hubble has been monitoring.”
Patrick Irwin, a planetary scientist at Oxford University, said the phenomenon is not a storm, as NASA described it in its release. Rather, it’s caused mainly—at least in our models—by a lowering of the methane abundance above the main cloud deck accompanied by a possible slight increase in the haze opacity.
Simon said, “The expanded Uranian polar feature was cool but more interesting to me is that bright storm just below it. That particular storm had flared up and was visible in small ground-based telescopes just prior to these observations, which shows how quickly they can change.”
Observing Neptune image, it gives the idea that a dark vortex has once again raised its ugly—yet captivating—head. The new anti- cyclonic tempest, seen at the best focal point of the photograph above, is around 11,000 kilometers (6,800 miles) over.
Simon said, “The Neptune dark spot is much larger than the one we saw a few years ago, and is comparable in size to the Voyager Great Dark Spot seen in 1989. This is also the first time we could see the region before a storm of that size formed, so that will help us in modeling the formation process.”